Protein interactions in chromosomes are being studied with crosslinking reagents and chemotherapeutic agents. A method has been developed for isolating and characterizing proteins crosslinked to DNA. Nitrogen mustard was found to crosslink certain nonhistone proteins, and possibly one histone to DNA. Histone-histone interactions are also being studied with crosslinking reagents. Water soluble carbodiimide crosslinks histones in chromosomes to one another, the major product being a H3-H4 dimer. Other products are being characterized to obtain some insight into histone-histone interactions.